Phenomenology of Low Quantum Gravity Scale Models

TL;DR

This work analyzes how low quantum gravity scale models from M-theory vacua—notably Type I string setups and Hořava–Witten compactifications—can yield viable unification and phenomenology with $M_s$ substantially below the four-dimensional Planck scale. It surveys multiple unification mechanisms, including conventional, rational, accelerated, and logarithmic-threshold scenarios, and highlights how brane/bulk geometries modify running couplings and thresholds. The paper also discusses SUSY-breaking mediations (gravity, gauge, Scherk–Schwarz) and several phenomenological implications, such as dark matter on a hidden wall, neutrino masses, axion scales, and ultra-high-energy cosmic rays, arguing for a preferred intermediate string scale around $10^{10}$–$10^{14}$ GeV. Overall, it argues that low-scale quantum gravity can be consistent within diverse M-theory frameworks and motivates further detailed model-building at the intersection of unification, SUSY breaking, and cosmology.

Abstract

We study some phenomenological implications of models where the scale of quantum gravity effects lies much below the four-dimensional Planck scale. These models arise from M-theory vacua where either the internal space volume is large or the string coupling is very small. We provide a critical analysis of ways to unify electroweak, strong and gravitational interactions in M-theory. We discuss the relations between different scales in two M-vacua: Type I strings and Ho\v rava-Witten supergravity models. The latter allows possibilities for an eleven-dimensional scale at TeV energies with one large dimension below separating our four-dimensional world from a hidden one. Different mechanisms for breaking supersymmetry (gravity mediated, gauge mediated and Scherk-Schwarz mechanisms) are discussed in this framework. Some phenomenological issues such as dark matter (with masses that may vary in time), origin of neutrino masses and axion scale are discussed. We suggest that these are indications that the string scale may be lying in the $10^{10} - 10^{14}$ GeV region.